We are searching for genes whose messenger RNAs are differentially produced during the lifespan of mice. Our approach to this analysis has been to make cDNA libraries from young (3 month) and old (27 month) mice from the GRC colony. These libraries have been pooled and analyzed by hybridization techniques to see what RNA messages are present in higher copy in young or old age. Several interesting cDNAs have been found. One clone whose mRNA is specific to young liver and falls about 1O-fold with age has been shown to be alpha-2-urinary globulin by DNA sequence analysis; this gene has already been shown by others to have this pattern of expression. Another clone was shown to be creatinine kinase, and its message was found to increase 3-4 fold with age in the brain. Two other clones have mRNAs that increase 3-5 fold with age, one spleen-specific and one liver-specific.